docs: def _lookup_in_all_namespaces(self, symbol):
          """Helper for lookup_symbol that looks for symbols in all namespaces.
  
          Args:
            symbol: Symbol
          """
          namespace = self.namespaces
          # Create a stack of namespaces.
          namespace_stack = []
          for current in symbol.namespace_stack:
              namespace = namespace.get(current)
              if namespace is None or not isinstance(namespace, dict):
                  break
              namespace_stack.append(namespace)
  
          # Iterate through the stack in reverse order. Need to go from
          # innermost namespace to outermost.
          for namespace in reversed(namespace_stack):
              try:
                  return self._lookup_namespace(symbol, namespace)
              except Error:
                  pass
          return None